Superconductors
Materials that allow electricity to flow without any resistance are known as superconductors. Such a lack of resistance occurs only in certain metals, alloys, and ceramics at very low temperatures. The temperature beneath which a particular material exhibits superconductivity is termed its transition temperature or critical temperature (TC). The earliest known superconductors possess critical temperatures very near absolute zero, but the first of a group of materials that transition into a superconducting state gradually and at significantly higher temperatures was developed in 1930, though its producers did not realize the importance of what they had created. The former variety of superconductors is commonly known as type 1 and the latter is termed type 2.
Mercury-Based Ceramic Superconductor
The phenomenon of superconductivity was first witnessed by Heike Kamerlingh Onnes in 1911. A Dutch physicist that founded the Cryogenic Laboratory at the University of Leiden, Onnes found that when he cooled a mercury rod to the boiling point of helium (4.2 Kelvin), the resistance of the rod suddenly dropped to zero. Onnes also discovered that superconducting materials can be forced into a non-superconducting state by exposing them to a magnetic field of sufficient strength. In 1913, the Nobel Prize for Physics was awarded to Onnes for his work in this area.
About 20 years after Onnes began his systemic study of superconductivity, additional information regarding the behavior of superconductors was revealed by a pair of German physicists. Walther Meissner and Robert Oschenfeld discovered that when a superconductor is placed in a relatively weak magnetic field, the field penetrates only a very thin layer of the material (the penetration depth of superconductors varies but is measurable in nanometers) before quickly decaying to zero. This phenomenon, which is unique to superconductors, is commonly known as the Meissner effect or the Meissner-Oschenfeld Effect. The Meissner effect is similar to perfect diamagnetism, but is distinct from it.
According to Lenz’s law, an induced electromotive force produces a current that induces a counter magnetic field that opposes the magnetic field generating the current. In a perfect diamagnet, the magnetic field lines produced exactly mirror those of the changing magnetic field that induce them, and thus repel the applied field. Superconductors exhibit this behavior, but in addition are capable of repelling a stable magnetic field. For instance, if a material contains a stable magnetic field in its interior and then the material is cooled below its critical temperature so that it enters a superconducting state, the stable magnetic field is cancelled out.
Yttrium-123
Depending on the strength of the magnetic field to be repelled, the Meissner effect varies in completeness. Sometimes the Meissner effect is so strong that a magnet can easily be levitated above a superconductor. Under other conditions, however, the effect begins to break down. The manner in which this breakdown occurs is one of the key variables that separate type 1 superconductors from type 2 superconductors. When the strength of a magnetic field applied to a type 1 superconductor exceeds a certain critical value (HC), the Meissner effect suddenly disappears. Any intermediate state produced in a type 1 superconductor, in which some regions contain a normal magnetic field and others contain no field, is a function of the shape of the superconductor rather than simply magnetic field strength.
Type 2 superconductors, on the other hand, can achieve an intermediate state based simply on the strength of an applied magnetic field. These types of superconducting materials each possess two critical field strength values (HC1 and HC2). When the magnetic field applied to a type 2 superconductor is greater than HC1 but less than HC2, a variable amount of magnetic flux penetrates the superconducting material, which continues to exhibit a lack of resistance to flowing electrical current. The closer the magnetic field strength is to HC2, the greater the amount of penetration. When HC2 is reached, the superconductivity of the material, including the Meissner effect, abruptly comes to an end.
Ever since superconductors were first discovered, scientists have attempted to explain their existence. The first theory of superconductivity to gain general acceptance was developed by American physicists John Bardeen, Leon Cooper, and Bob Schrieffer. According to BCS theory, which was named in honor of its developers, the interaction between a superconductor’s crystal lattice and electrons is such that at very low temperatures the electrons form bound pairs, often referred to as Cooper pairs. The pairing is a consequence of minor attractive forces between the electrons related to vibrations of the lattice and a process known as phonon-mediated coupling.
Superconducting Thick Film (Yttrium-123)
Paired electrons behave quite differently than single electrons. The pairs are somewhat similar to bosons, which are particles that do not obey the Pauli exclusion principle and are able to condense into the same energy state as other bosons. Pairs of electrons exhibit a small energy gap that impedes the collisional interactions that normally result in electrical resistance. Under conditions in which the band gap is greater than the thermal energy, a complete lack of resistance can be observed. In 1972, Bardeen, Cooper and Schrieffer were awarded the Nobel Prize for Physics in honor of their theoretical explanation of superconductivity involving paired electrons. The BCS theory remains a keystone in the modern understanding of type 1 superconductors, but a satisfactory or generally accepted explanation of type 2 superconductors has yet to be developed.
A lead bismuth alloy was the first known type 2 superconducting material ever produced. In 1936, six years after that material was developed, Lev V. Shubnikov at the Kharkov Institute of Science and Technology in the Ukraine became the first scientist to recognize two distinct critical magnetic fields in a superconducting material, formally marking the realization that there existed more than one type of superconductor. Type 2 superconductors did not become a major focus of scientific attention, however, until half a century later. The announcement in 1986 that two IBM research scientists, Georg Bednorz and Alex Müller, had developed a ceramic type 2 material that became superconducting at 30 K sparked a flurry of new superconductor studies around the globe. Researchers rapidly synthesized a number of other superconducting ceramics with even higher critical temperatures, almost immediately producing materials that superconduct at 40 K and then 50 K.
A tremendous breakthrough in the superconductor field occurred in the early spring of 1987. At that time, Paul Chu and his colleagues at the University of Houston in Texas developed a perovskite ceramic material that became a superconductor at the then incredibly high temperature of about 90 K. Such an improvement in critical temperature was especially significant because it enabled materials to be transitioned into a superconducting state with liquid nitrogen, a widely available and relatively inexpensive coolant.
Praseodymium-123 Single Crystal
As the race to achieve higher and higher critical temperatures continued, many new superconducting materials were developed. Soon a critical temperature of about 125 K was reached, and some optimists began to envision a world in which high temperature superconductors were commonly employed in new and vastly improved technological devices. However, advances in this area began to slow in the following years. In just a few years, there had been a more than a 100-Kelvin increase in the attainable critical temperature, but only modest increases have been achieved more recently. As of late 2005, the world record for the highest critical temperature was 138 K. The material holding the record is a thallium-doped, mercuric-cuprate containing the elements mercury, thallium, calcium, barium, copper and oxygen. It can be forced into exhibiting an even higher critical temperature when placed under extreme pressure.
With critical temperatures becoming more difficult to improve, other aspects of superconductor science have gained more significant interest and important advances in various areas have been made. For example, scientists discovered in 2000 the first high temperature superconductor that lacks copper and a year later the first perovskite superconductor comprised only of metals. In addition, 2001 saw the discovery that magnesium diboride, which had been synthesized many years earlier in Japan, was a superconductor with a substantially higher TC (39 K) than any other elemental or binary alloy superconductor yet produced. The unique properties of magnesium diboride render it well suited for certain applications, such as magnetic resonance imaging (MRI). Even more recently, in 2005, researchers found that increases in the critical temperatures of layered perovskite ceramics can frequently be achieved via escalation of the weight ratios of alternating planes within the materials, creating the possibility of a renewed race toward superconductors that have zero resistivity at room temperatures.
Though room temperature superconductors would greatly broaden the spectrum of potential uses for superconductivity, they currently remain in the realm of science fiction. Yet, some superconducting materials have already found their way into various industries. In 1989, the first company to commercially produce high temperature superconductors was formed. The company was then known as Illinois Superconductor, but has since been reorganized into ISCO International. Their initial focus was on providing depth sensors based on superconducting technology that could be used in medical equipment at liquid nitrogen temperatures.
2212 Ceramic Superconductor Thick Film
Since the pioneering work of Illinois Superconductor, many other groups, agencies, and companies have entered the arena of superconductor-based products and services. For instance, a team of scientists at the Korea Research Institute of Standards and Science developed a double-relaxation oscillation superconducting quantum interference device referred to as SQUID. The device has been employed in magnetoencephalography (MEG), a non-invasive means of mapping the brain, and in various military applications, such as the detection of submarines by the United States Navy. The military has pioneered the use of superconductors for other applications as well. In 2003, the US military employed an e-bomb, which is a superconductor-based device that can produce a high-intensity electro-magnetic pulse (EMP) strong enough to disrupt targeted electronic equipment, for the first time during a war. Also, superconducting wire and tape have begun seeing use in the motors of Navy ships, which are much smaller than conventional motors, and may eventually be successfully employed to build smaller antennas for submarines.
Power companies have also played a significant role in moving superconductors toward profitable and widespread employment. Since electric generators utilizing superconducting wire are much more efficient than generators made with copper wire, power companies stand to make significant gains through their use. In 2002, General Electric Power Systems received more than 12 million dollars from the US Department of Energy to help spur superconducting generator technology toward complete commercialization. Another area in which power utilities have been experimenting with superconductor usage is power transmission. Because of the prohibitive cost of cooling the many miles of wire that carry power from a station to various cities, however, superconductor-based transmission lines are still in an early testing phase in which they span only short distances. Superconductor-based transformers and fault limiters are examples of power-related superconductor technology that are at a more advanced stage of development.
Superconductors may revolutionize many other industries in the future as well. Computers, motor vehicles, and high-speed magnetic levitating trains are just a few of the many devices that could be greatly advanced through the use of superconducting materials. Many obstacles, such as the brittleness of superconducting ceramics and the need for expensive cooling systems, remain that will likely have to be overcome before superconductors become integrated into the daily lives of humans. Yet it was not long ago when the microprocessor was itself not much more than science fiction, but now the technology drives many everyday devices, such as cellular phones, computers, and digital microwaves. Many predict that superconductors will follow a similar path, so that future generations may one day have difficulty envisioning what life would be like without them.
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